The Australian Transport Safety Bureau (ATSB) is Australia's national transport safety investigator. The ATSB's function is to improve safety and public confidence in the aviation, marine and rail modes of transport. The ATSB is Australia's prime agency for the independent investigation of civil aviation, rail and maritime accidents, incidents and safety deficiencies.

What happened

On 11 November 2012, an Emirates A380 aircraft, registered A6-EDA, departed Sydney Airport for Dubai, United Arab Emirates. While climbing through an altitude of approximately 9,000 ft, the crew reported hearing a loud bang, accompanied by an engine No 3 exhaust gas temperature over-limit warning. Shortly thereafter, the engine went through an uncommanded shut down. The crew jettisoned excess fuel and returned the aircraft to Sydney for a safe landing and disembarkation of the passengers and crew.

What the ATSB found

The investigation found that the increase in the exhaust gas temperature and subsequent engine shut down was a result of significant internal damage that had initiated within the high pressure turbine (HPT) module. The damage had resulted from the effects of HPT stage-2 nozzle distress, likely caused by exposure to hotter than expected operating temperatures. The nozzle distress led to eventual failure and separation into the gas flowpath. Over the preceding weeks there were two other engines within the operator’s fleet that had experienced a similar problem, and a number of steps had been taken by the manufacturer to address the issue, including the increased monitoring of distress development. During the previous flight, the engine health and trend monitoring program had identified a performance trend shift with this particular engine, and it was due to be inspected upon return to the main base in Dubai.

What's been done as a result

The engine manufacturer, Engine Alliance, had issued a service bulletin in June 2010 for the replacement of affected HPT stage-2 nozzle segments with new, more durable components during the next workshop visit when the HPT stage-2 was removed from the engine. Following this occurrence, another service bulletin was released on 6 December 2012, requiring the direct inspection of the nozzle segments that had not yet been replaced. The US Federal Aviation Administration also released an Airworthiness Directive which required inspection of the nozzle segments and their removal from service if distress was identified.

As the nozzle degradation mechanism was an emerging issue for the engine manufacturer at the time of the occurrence, the information and experiences associated with this occurrence have been used to refine and improve the trend monitoring program. Under the new limits set, this engine would have been inspected two flights prior to the occurrence flight. At the time this report was released, the manufacturer was continuing work to better understand the initial onset of nozzle distress and potential for further design improvements.

Safety message

While the distress to the HPT was severe enough in this case to result in an in-flight engine shutdown, the associated risks to the safety of continued flight were relatively low, given the failure had been contained and the operator’s procedures were effective in managing the engine shut down. This occurrence also pointed to the value of real-time engine condition monitoring, since advanced warning of engine degradation and efficiency loss allows inspection and corrective action before damage progresses to a level where it can cause an in-flight shut down.

The threshold limits for the engine trend monitoring program were not set at a level that provided sufficient opportunity for inspection of the engine before failure could occur from the effects of HPT stage-2 nozzle degradation.